1. Field of the Invention
The present invention relates to a switching power supply apparatus.
2. Description of the Related Art
In recent years, among small sized information terminals such as mobile telephones, PDAs (Personal Digital Assistants) and the like, there are devices in which a voltage is required that is higher than an output voltage of a battery, as in, for example, an LED (LightEmittingDiode) used as a backlight for liquid crystal or the like. For example, in many of these small sized information terminals, Li-ion batteries are used; the output voltage thereof is normally about 3.5 V, and about 4.2 V when fully charged, but the LED requires, as a drive voltage thereof, a voltage higher than the battery voltage. In cases in which a voltage higher than the battery voltage is required, the battery voltage is stepped up using a voltage boost circuit such as a switched capacitor system or the like, and the voltage required to drive a load circuit, such as the LED or the like, is obtained. In addition, there are cases in which a negative power supply is necessary in these types of small sized information terminals; in these cases also, a desired negative voltage is obtained using a voltage inverting circuit of a switched capacitor system (Patent Document 1).
Here, a case is considered in which, for some small sized information terminal, a load circuit in which a voltage is necessary that is higher than the battery voltage, and a load circuit in which a negative voltage is necessary, are driven at the same time. In order to supply a suitable voltage to the load circuits that have to be driven with different voltages in this way, a method may be considered in which a plurality of charge pump circuits are connected in series, and output of each of the charge pump circuits is supplied to the respective loads. For example, in cases in which a charge pump circuit that has a charge-pump ratio of two, and a voltage inverting type charge pump circuit are connected in a two step series, if the battery voltage is written as Vbat, it is possible to supply three different voltages, Vbat, 2×Vbat, and −2×Vbat, to the loads.
Patent Document 1: Japanese Patent Application Laid Open No. 2001-258241
In general, a charge pump circuit is provided with a flying capacitor, an output capacitor, and a plurality of switch elements connecting these two capacitors; by controlling the ON-OFF state of the switch elements, a charge is stored in the flying capacitor and the output capacitor, and input voltage is converted into a desired voltage.
Here, as described above, cases are considered in which a plurality of charge pump circuits are connected in series. In these cases, the input voltage of the charge pump circuit on a second stage is the output voltage of the charge pump circuit on a first stage, and the flying capacitor of the charge pump circuit on the second stage is charged by a charge stored in the output capacitor of the charge pump circuit of the first stage. Accordingly, in each charge pump circuit, if a charge-discharge state of a capacitor, that is, an ON-OFF state of the switch elements, is distinctly and independently controlled, according to the case, a ripple in the output voltage becomes large.